1. Field of the Invention
The present invention relates to a casting method and a casting apparatus, and more particularly to a casting method and a casting apparatus in which a cast product having a desired shape is cast by allowing molten metal poured into an cavity of a molding die and a reducing compound to be contacted with each other whereby an oxide film formed on a surface of the above-described molten metal is reduced.
2. Description of the Related Art
There exist various types of aluminum casting methods such as, for example, a modified aluminum casting method proposed in Japanese Patent Application No. 108078/2000 by two inventors of the present application.
A molding die to be adopted by this modified aluminum casting method is shown in FIG. 8. The molding die 100 thus shown in FIG. 8 is such a molding die made of metal as is used in a gravity casting method; on this occasion, the molding die is of a separate type comprising a lower die 102a and an upper die 102b. By these dies 102a and 102b, formed is a cavity 104 in which a cast product having a desired shape is cast.
Further, in the upper die 102b, a feeder head portion 108 is formed between a sprue 106 from which molten metal of aluminum or an alloy thereof is poured and the cavity 104, and also air-vent holes 110 from which an air in the cavity 104 is discharged when the molten metal is poured into the cavity 104 is formed.
In the improved aluminum casting method using such molding die 100, after a reducing compound, that is, a magnesium-nitrogen compound (Mg3N2) is introduced into the cavity 104 of the molding die 100, the molten metal of aluminum or the alloy thereof is poured into the sprue 106 of the molding die 100 and, then, the molten metal is filled in the cavity 104 and the feeder head portion 108 while the air is discharged from the air-vent holes 110.
Next, the molten metal in the cavity 104 is solidified by cooling the molding die 100 in which the molten metal is filled in the cavity 104 and the like as it stands still. A void which is caused by shrinkage with solidification of the molten metal is supplemented by allowing a part of the molten metal in the feeder head portion 108 to be flowed down in the cavity 104.
The improved aluminum casting method is a reduction casting method in which an oxide film formed on a surface of the molten metal of aluminum or the alloy thereof is reduced in the presence of a reducing compound within the cavity 104 of the molding die 100 to decrease a surface tension of the molten metal and, as a result, a flowing property and a running property of the molten metal can be enhanced.
For this feature, in the improved aluminum casting method, coating of a coating agent which is to be coated on surfaces of inner walls of the feeder head portion and the cavity aiming for enhancement of a flowing property and the like of the molten metal and the like on which the oxide film is formed can be omitted thereby enabling to promote a reduction of production steps and enhance a transferring property of the molding die 100.
Now, depending on the shapes of the cast products, there is a case in which the cavity 104 of the molding die 100 is forced to have a shape where a narrow portion having a smaller cross-sectional area than that of a terminal portion is formed halfway between the sprue and the terminal portion. For example, there is a case in which the cavity 104 is forced to have a shape where a first cavity portion 104a in which a molten metal inlet of the cavity 104 is arranged and a second cavity portion 104b, that is, the terminal portion are connected with a narrow portion 104c which is formed narrower than the first cavity portion 104a and the second cavity portion 104b (hereinafter, also referred to only as cavity portion 104a and cavity portion 104b respectively, or as cavity portions 104a and 104b collectively).
In the cavity 104 shown in FIG. 9, after the reducing compound, that is, the magnesium-nitrogen compound (Mg3N2), is introduced into the cavity 104 of the molding die 100, the molten metal of aluminum or the alloy thereof poured into the sprue 106 is then poured into the first cavity portion 104a and, thereafter, poured into the second cavity portion 104b via the narrow portion 104c. Such pouring, i.e., filling of the molten metal in the cavity 104 is performed in a short period of time by allowing an oxide film formed on the surface of the molten metal to be reduced in the presence of the reducing compound.
However, since the molten metal filled in the narrow portion 104c of the cavity 104 is smaller in quantity than that in the cavity portions 104a and 104b and faster in cooling rate than that filled in the cavity portions 104a and 104b, the molten metal filled in the narrow portion 104c is solidified earlier than that filled in the second cavity portion 104b. 
For this reason, even when the void is formed while shrinkage is generated with the solidification of the molten metal filled in the second cavity portion 104b, the second cavity portion 104b is not replenished with the molten metal filled in the first cavity portion 104a and the feeder head portion 108, that is, an effect of feeding the molten metal can not be expected whereupon there is a fear that a shrinkage hole or the like may be generated in an obtained cast product.
Meanwhile, though it is possible to solve the shrinkage hole or the like to be generated with the solidification of the molten metal filled in the second cavity portion 104b by independently arranging the feeder head portion in each of the cavity portions 104a and 104b, such an arrangement as forms feeder head portions in a plurality of different places will lead to a complexity of a constitution of the molding die.
Further, since a part of the molten metal which is solidified in the feeder head portion 108 is not a cast product, the portion is cut off to be disposed. Even when it is considered that the thus-cut off portion is reused after being melted again, a loss of energy must be expected.
Therefore, forming feeder head portions in a plurality of different places increases a capacity of a part of non-cast product, decreases a yield of the cast product of the molten metal poured into the molding die 100 and, accordingly, increases a loss in workability and energy.
Under these circumstances, an object of the present invention is to provide a casting method and a casting apparatus in which, when casting is performed using a molding die in which a number of a feeder head portion to be formed between a sprue and a cavity having a complicated shape is allowed to be as small as possible, a shrinkage hole or the like which is caused by shrinkage with solidification of the molten metal filled in the cavity and which is generated in an obtained cast product can be prevented.
As a result of an extensive study made by the present inventors to solve the above-described problems, it has been found that, in a reduction casting method which allows a reducing compound to be preliminarily present in a cavity 104 of a molding die 100 (shown in FIG. 8), a cooling rate of molten metal filled in a feeder head portion 108 and a narrow portion 104c of the cavity 104 can be made slower by coating a coating agent having a heat insulating effect only on surfaces of inner walls of the feeder head portion 108 and the narrow portion 104c of the cavity 104, compared with a case in which the surfaces of the inner walls of the feeder head portion 108 and the narrow portion 104c of the cavity 104 are not coated by the coating agent.
As described above, the present inventors have found that the shrinkage hole or the like which is caused by shrinkage with solidification of the molten metal filled in the second cavity portion 104b of the cavity 104 and which is generated in an obtained cast product can be prevented by allowing the feeder head portion 108 and the narrow portion 104c of the molding die 100 to have a higher heat insulating property than other portions of the molding die 100 to attain the present invention.
Namely, according to the present invention, there is provided a casting method for casting a desired shape of a cast product by allowing molten metal poured into a cavity of a molding die and a reducing compound to be contacted with each other while reducing an oxide film formed on a surface of the molten metal, comprising the steps of:
using the molding die in which a feeder head portion is arranged between a sprue from which the molten metal is poured and the cavity and a difference of heat insulation is partially provided between the feeder head portion and the cavity such that the molten metal filled in the cavity and the feeder head portion is sequentially solidified in a direction of from a terminal portion of the cavity to the feeder head portion; and
replenishing the cavity with at least a part of the molten metal filled in the feeder head portion, when a void is formed by shrinkage with solidification of the molten metal filled in the cavity.
Further, according to the present invention, there is provided a casting apparatus for performing a casting while an oxide film formed on a surface of a molten metal is reduced by allowing the molten metal and a reducing compound to be contacted with each other, comprising:
a molding die having a cavity for receiving the molten metal, a sprue from which the molten metal is poured and a feeder head portion arranged between the sprue and the cavity,
wherein a difference of heat insulation is partially provided between the feeder head portion and the cavity such that the molten metal filled in the cavity and the feeder head portion is sequentially solidified in a direction of from a terminal portion of the cavity to the feeder head portion.
The present invention can preferably be adopted, when the molding die comprising the feeder head portion, arranged between the sprue from which the molten metal is poured and the cavity, and the cavity in which a narrow portion that has a smaller cross-sectional area than the terminal portion is arranged halfway between an inlet, which is in a side of the feeder head portion, of the cavity connected with the feeder head portion and the terminal portion thereof, wherein the feeder head portion and the narrow portion are formed such that they have a higher heat insulating property than the terminal portion, is used.
On this occasion, a difference of heat insulation can easily be provided between the feeder head portion and the terminal portion of the cavity by forming a part of the molding die, in which the feeder head portion is arranged, by a material that has a higher heat insulating property than a material that forms the terminal portion of the cavity of the molding die.
Further, a difference of heat insulation can easily be provided between the narrow portion and the terminal portion even in the cavity by forming a part of the molding die, in which the narrow portion of the cavity is arranged, by a material that has a higher heat insulating property than a material that forms the terminal portion of the cavity.
On the other hand, a difference of heat insulation can easily be provided between the feeder head portion and the narrow portion of the cavity, and the terminal portion of the cavity by using the molding die in which a heat insulating treatment, such as an application of a heat insulating coating agent or the like that is non-reactive to a reducing compound which contacts the molten metal, is performed on a surface of an inner wall of each of the feeder head portion and the narrow portion of the cavity, and the heat insulating treatment is not performed on a surface of an inner wall of the terminal portion of the cavity.
Further, a part of the molding die, in which the feeder head portion is arranged, can be used as a common member by using the molding die in which a part of the molding die, in which the feeder head portion is arranged, is constructed such that the part is detachable from a cavity portion of the molding die.
According to the present invention, when molten metal of aluminum or an alloy thereof is used as the molten metal, a magnesium-nitrogen compound which is obtained by allowing a magnesium gas and a nitrogen gas to be reacted with each other as raw materials can preferably be used as the reducing compound.
Further, blocking or the like by the reducing compound in a halfway of an introducing passage leading to the cavity can be prevented by arranging a molten metal-introducing passage that introduces the molten metal into the feeder head portion and an introducing passage that introduces a raw material of a reducing compound into the cavity such that the reducing compound is generated in the cavity in a part of the molding die in which the feeder head portion is arranged.
In the present invention, a difference of heat insulation is partially provided in the feeder head portion and the cavity such that the molten metal filled in the feeder head portion, that is formed between the sprue from which the molten metal is poured and the cavity, and the cavity is sequentially solidified in a direction of from a terminal portion of the cavity to the feeder head portion
For this provision, when the molten metal is sequentially solidified in a direction of from the terminal portion of the cavity to the feeder head portion and a void is formed in the cavity caused by shrinkage with solidification of the molten metal, a part of the molten metal filled in the feeder head portion is flowed into the cavity for replenishment, that is, the effect of feeding the molten metal is secured until the molten metal filled in the cavity is fully solidified and, as a result, the shrinkage hole or the like to be generated in the cast product to be obtained can be prevented.